The morphology of single GNP unities and their aerogels was investigated by scanning electron microscopy (SEM). The SEM micrograph of GNP is given in Figure
a. The petal-shaped unities, shown in Figure
a, have two main dimensions of ca
. 80 μm and a thickness of only a few tens of nanometer. As visible in Figure
b, these petal-like structures are randomly distributed in the aerogel bulk, and a very porous solid results.
SEM micrographs showing the morphology of the graphite nanoplatelets (a) and the GNP aerogel (b).
shows the X-ray diffraction (XRD) diffractogram of a graphite nanoplatelet sample. According to the Scherrer equation, the average GNP thickness is 15 nm.
XRD diffractogram of the graphite nanoplatelet sample.
Graphite nanocrystals are much more chemically reactive than the ordinary graphite flakes; consequently, a number of graphite derivatives can be easily prepared using such nanoscopic graphite crystals as reactant (for example, graphite nanoplatelets can be quantitatively and quickly converted to graphite oxide by the Hummers method [10
]). The free radical addition to the carbon-carbon double bond is a typical reaction involving benzene (C6
) and other polycyclic aromatic compounds; as a consequence, graphene, fullerenes, carbon nanotubes, and other nanostructures based on the sp2
carbon could also give the same type of reaction. Therefore, the chemical cross-linking of graphite nanoplatelets could be based just on this type of reaction, but a bi-radical molecule should be used in order to graft simultaneously two GNP unities. Elemental sulfur is made of S8
rings, which is converted into a linear polymeric bi-radical molecules (·S-S6
-S·) at a temperature of 160°C; such reaction is known as λ-transition. The λ-transition of elemental sulfur is an endothermic process which is clearly visible in a DSC thermogram [11
]. In particular, the DSC thermogram of elemental sulfur contains three endothermic signals: (1) the α → β transition of the sulfur crystals at 98°C, (2) the melting of the β-crystals at 116°C, and (3) the λ-transition at 160°C (see Figure
(thermogram a) and Table
DSC thermograms of the S/GNP system. First (thermogram a) and second (thermogram b) heating run.
Thermodynamic properties of the S/GNP system obtained by DSC
The isothermal annealing of the reactive sulfur/GNP system at temperatures higher than 160°C allows a more or less complete conversion of polysulfur bridges (C-S8-C) to monosulfur bridges (C-S-C) which are sort of electrical connections between the graphene planes because conjugation is possible through the sulfur atom. When the GNP-based aerogels are devoted to electrical applications (e.g., electrodes for batteries and supercapacitors, electrolysis cells, etc.), such type of chemical cross-linking results are extremely convenient.
The λ-transition is characterized by a clearly visible endothermic signal (the enthalpy change is 1.10 J/g), and it can be detected also in the DSC analysis of S/GNP mixtures (see Figure
(thermograms a and b)). Consequently, important information on the chemical interaction between sulfur and GNP can be obtained by DSC analysis.
In particular, the change of the S-S bond concentration (i.e., the [S-S]/[S-S]0 value) can be calculated by analyzing the change in the enthalpy variation of the λ-transition signal. In particular, the thermal treatment of the S/GNP systems significantly modifies the DSC thermogram: the melting peak of the β-sulfur at 116°C disappears, and the λ-transition peak results strongly decreased because the [S-S] is proportional to ΔH of the λ-transition. Such decrease of the λ-transition peak depends on time and temperature of the thermal annealing treatment. The fraction of reacted S-S bonds (α) is given by the following expression:
The temporal evolution of α
at two different temperatures (300°C and 350°C) is shown in Figure
. As visible, the experimental data are well described by an exponential recovery function (i.e., α
Behavior of the reacted S-S bond fraction with time. The experimental data points have been fitted by the exponential recovery law.
Such experimental behavior of the reaction conversion suggests the following three-step reaction mechanism:
The first reaction step involves the cleavage of the S-S bond with the formation of two sulfur radicals. This elemental reaction is reversible and has a slow specific rate. In the second elemental reaction, one of the two sulfur radicals is added to the carbon-carbon double bond with the formation of S-C bond and one carbon radical. Such reaction should have a fast rate because an unstable reactant (the sulfur radical) is involved. In the last elemental reaction, the carbon radical combines with the second sulfur radical with the formation of a new S-C bond. Also, this step should be very fast because the combination of two radicals is involved. The full reaction rate depends only on the slowest step which is characterized by a first-order kinetic; consequently, the rate expression is −d[S-S]/dt = k[S-S], which after integration provides an exponential recovery law (α = 1 − e−kt). Finally, according to the DSC analysis, the S/GNP chemical interaction is of the first kinetic order, and the involved mechanism is a direct reaction between the sulfur radicals generated at λ-transition and the sp2 carbon atoms located at the edges of the graphite nanocrystals.
In order to establish the temperature dependence of the reaction conversion, the rate constant of the reaction has been evaluated at different temperatures, giving for example the following values:
and these values have been used to evaluate the constants in the Arrhenius law:
In particular, the activation energy of the reaction (46.9 kJ/mol) is in the same order of magnitude as a chemical bond (the S-S bond energy is ca
. 213 kJ/mol). The behavior of the reaction conversion (α
) under conditions different from that experimentally evaluated can be obtained by a simulation (the temperature values can be both interpolated or extrapolated). In Figure
, the following expression has been used: α
)] with αmax
= −0.454 + 3.86 × 10−3
(°C) (a linear behavior has been assumed for the αmax
). As visible in Figure
, a conversion degree close to 100%, which corresponds to a complete formation of monosulfur bridges (C-S-C), is possible only at a temperature higher than 350°C for a time period longer than 300 min.
Theoretical behavior of the time dependence of α at different temperatures.
The S/GNP chemical interaction was also investigated by thermogravimetric analysis. In particular, during the heating run (at 10°C/min) of a S/GNP sample (50% by weight of sulfur), some of the elemental sulfur reacts with carbon and bonds at GNP edges. In fact, such sulfur fraction cannot evaporate also at temperatures higher than the pure sulfur boiling point (444°C), and a residual sulfur content (ca
. 30% by weight) results in the material, as visible in the TGA thermogram shown in Figure
TGA thermogram of S/GNP mixture (50% by weight of sulfur).
It has been found that mechanically resistant GNP aerogels resulted after a cross-linking treatment with elemental sulfur at 350°C for 3 h (see Figure
). A large number of electrically conductive monosulfur bridges should be generated in these conditions, and a good electrical conductor results (with resistivity of 3 Ω cm).
Fragile structure of the GNP aerogel (a) results mechanically stabilized by treatment with elemental sulfur (b).